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<!-- <script>
function fun(){
    //var str1;
    console.log(str1);//undefined
    var str1='123455';
    console.log(str1);//123455
}
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<script>
//var num;
console.log(num);//undefined
var num=11;
console.log(x);//undefined
if(false){
    alert(1);
    var x='nihao';
}
console.log(x);//undefined
</script>
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